The assessment of net ecosystem CO2 exchange often relies on eddy covariance measurements. Under stable, low-turbulence conditions, the measured flux may not be representative of the net ecosystem exchange (NEE), as unmeasured fluxes (e.g., advection) can become relevant. Consequently, such periods need to be filtered out for robust flux calculations. Typically, the focus lies on nighttime filtering alone, yet daytime flux measurements can also be unrepresentative. This study evaluates well-established and novel filtering methods applied both at nighttime and daytime at a mountain forest site in Tyrol, Austria (Forest-Atmosphere-Interaction-Research (FAIR) site, AT-Mmg). Established methods, including friction velocity filtering, its counterpart using the standard deviation of vertical velocity fluctuations (σw) and an after-sunset flux maxima approach (commonly referred to as van Gorsel method), are compared. Additionally, we use a more recent approach with a physically-derived measure of flow decoupling for filtering. Moreover, we introduce a novel K-means clustering approach that groups flow situations into clusters based on vertical profiles of temperature, σw and wind speed. Clusters in which the measured flux is expected to be a reasonable NEE estimate are retained. Such scenarios are Foehn periods, early-night situations with high turbulence and low stability, or well-mixed afternoon conditions. Despite being based on widely differing assumptions, the various filtering approaches yielded similar carbon budget estimates over 14 months of measurements (-224 to -286 g C m−2 for nighttime filtering and -440 to -382 g C m−2 for all-day filtering), in contrast to the unfiltered budget of -534 g C m−2. Nighttime filtering results in higher respiration rates throughout the night, while daytime filtering suggests increased morning carbon uptake compared to unfiltered data.

中文翻译：

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山林上空涡流协方差导出的净生态系统CO[公式略]交换的多种昼夜过滤方法的一致性


净生态系统二氧化碳交换的评估通常依赖于涡度协方差测量。在稳定、低湍流条件下，测量的通量可能不能代表净生态系统交换（NEE），因为未测量的通量（例如平流）可能变得相关。因此，需要过滤掉这些周期以进行稳健的通量计算。通常，重点在于夜间过滤，但白天通量测量也可能不具有代表性。本研究评估了奥地利蒂罗尔州山林站点（森林大气相互作用研究（FAIR）站点，AT-Mmg）在夜间和白天应用的成熟且新颖的过滤方法。比较了现有的方法，包括摩擦速度滤波、使用垂直速度波动标准差 (σw) 的对应方法以及日落后通量最大值方法（通常称为 van Gorsel 方法）。此外，我们使用了一种更新的方法，通过物理导出的流量解耦测量来进行过滤。此外，我们引入了一种新颖的 K 均值聚类方法，该方法根据温度、σw 和风速的垂直剖面将流动情况分组。保留测量通量预计为合理 NEE 估计值的簇。此类场景包括焚风时期、高湍流和低稳定性的深夜情况，或混合良好的下午条件。尽管基于差异很大的假设，各种过滤方法在 14 个月的测量中得出了相似的碳预算估计值（夜间过滤为 -224 至 -286 g C m−2，所有过滤为 -440 至 -382 g C m−2）。天过滤），与-534 g C m−2 的未过滤预算形成对比。 夜间过滤会导致整个晚上的呼吸速率更高，而白天过滤则表明与未过滤的数据相比，早晨的碳吸收量有所增加。